Tunnel kiln



May 31, 1932. E. \B.FoRsE ET AL TUNNEL KILN 3 Sheets-Sheet l Q -sw La IIIIIIIIW/ i INVENTORS aowm B. FORSE BY lcrkss RGEGER ATTORNEY Filed Jan. 1e, 19:51

May 3.1, 1932. E. B. FORSE ET AL TUNNEL INVENTORS Enwm e.. Ponsa CHARLE EGEIGER K WU ATTORNEY Filed Jan. 16, 1931 May 31, 1932. E. B. FORSE ET AL 3 Sheets-Sheet 3 EDWlN B. FORSE CHARLES FGENSER Filed Jan. 16, 1931' Fil.

ATTORNEY Patented ivfay 3l, 1932 UNITED STATES EDWIN B. FORSE AND CHARLES F.I GEIGER, OF METUCHEN, NEW JERSEY, ASSIGNORS T THE CARBORUNDUIVI COMPANY, 0F NIAGARA. FALLS, NEW' YORK, A CORPORATION OF PENNSYLVANIA TUNNEL KILN Application filed January 16, 1931.

This application relates particularly to tunnel kilns which are provided with combustion chambers of the radiating combustion type and with efficient means for controlling the rate of cooling of the Ware after it has been subjected to maximum temperature conditions in the tunnel kiln. The present application is a continuation in part of our eopending application, Serial No. 276,222, filed May 9, 1928, for tunnel kilns and method of operating the same, and deals particularlyY with the means and method for regulating the cooling of the ware after it has passed the firing zone.

Our invention and the method of operating the same may be readily understood by reference to the accompanying drawings which illustrate an embodiment of our invention, and in which:

Figure l is a plan view partly in section and partly in outline, of a kiln constructed in accordance with our invention;

Figure 2 is a longitudinal vertical section along the center of the kiln shown in Figure l;

Figure 3 is a transverse Vertical section on a larger scale in the plane of line III-III of Figure l;

Figure 4 is a similar section in the plane of line iV---IV of Figure l;

Figure 5 is a detail vertical section in the plane of line VV of Figure l;

Figure 6 is a similar View in the plane of line VI-VI of Figure l;

Figure 7 is a transverse section on a larger scale in the Figure l;

Figure 8 is a similar view in the plane of line VIII-VIII of Figure l, and;

Figure 9 is a transverse vertical section on a larger scale in the plane of line IX-IX of Figure l.

In Figures l and 2 the scale of the drawings as compared with a full sized kiln is so small as to render these figures merely more or less diagrammatic. In Figures 3 to 9 inclusive, the sections are on a relatively larger scale and the preferred details of the wall construction and the arrangement of the various refractories is more clearly apparent;

plane of linevVII-VII ofv Serial No. 509,065.

furthermore, in Figure l the portion of the kiln below the level of the radiating combustion chamber has not been shown, for the reason that this trackway is well understood 1n the art, and its arrangement is clearly shown in the cross sections through the kiln.

ln the drawings, the kiln which ,is shown generally in Figures l and 2, has a4 central tunnel which may be considered as divided into a number of zones, according to the type of ware which is to beltreated in the kiln. For certain types of ceramic ware the kiln preferably has three zones of progressively increasing temperatures, hereinafter designated Zone u, zone b, zone c, and followed by a cooling zone. In Figures 1 and 2 the ware travels through the kiln on cars which move along tracks 2 in the bottom of the tunnel 3. The direction of. travel of these cars is opposite to the direction of the arrows in Figures l and 2, the arrows indicating the direction of gas and air flowing through certain parts of the kiln, as hereinafter described. The first zone a may be considered the first part of the kiln which the ware enters and extends from point A to point B in Figures l and 2. The next zone b ex tends from point B to point C. The third heating zone c ext-ends from point C to point D, and from D to E is the cooling zone. Beyond the cooling zone DE is a further eoolling zone with which metal walls may be use( In the second zone b, at each side of the kiln and preferably directly opposed to each other, are heating units which ably of the well known radiating combustion type. These combustion units, designated 4, are formed of a silicon carbide refractory.

In the last heating zone c Where, ordinarily the final burning or firing of theware takes place, relatively high temperatures must be secured. Located in this zone at each side of the tunnel 3 and supported on ledges 5, as best shown in Figure 4, are one or more radiating combustion chambers 6 of a construction similar to the radiating combustion chamber 4, but capable of burning considerably more fuel.

In Figures l and 2 we have shown the kiln as being provided with two of the combustion units 6, on each side of the tunnel, the combustion units being turned in opposite directions. Each combustion unit 6 1s provided with a fuel port 7` The cooling zone of the kiln is of such construction (see Fig. 8) that the rate of cooling can be regulated (insofar as this is possible with the material of which the side wall 8 is constructed) by means of controlled variations in the rate of flow of cooling air through air ducts 10. The air may be supplied from a blower 47 which drives air through a duct 48 which is provided with a damper 49. Branch ducts lead from the main duct 48 to thc ducts of each section of the cooling chamber. The walls 8 ot the cooling chamber are made of a refractory material whose thermal conductivity is in excess of 0.006 calorie/cm.3/sec./OC. and of al thickness preferably not exceeding nine inches. The thickness of the wall 8 may be much less than this if the material of which it is composed retains its mechanical strength when heated to temperatures which may reach values exceedingA 1200o C. in the portion of the cooling zone adjacent to the firing chamber. Assuming the thermal conductivity of ireclay as 1, the following numbers give the order of magnitude of the thermal conductivities of some other refractory materials and also the order of magnitude of some other physical properties which should be considered in selecting the material of which the wall 8 is to be constructed. It is understoodthat these values vary with the temperature.

Silicon Fused alu-i i Carbide mina i Magnesite Clay l Thickness Same Same Same Same Thermal conductivity.. 9 2. 5 2. 5 i Transverse strength at 1350Q C.,lbs./in.7 l500 200 l 140 200 Crushing strength at y 1350a C., lbS./in.7 1000 200 ll 50 Resistance to spelling. l it 0. 3 l Fusion point 2240" C. IU00 C. i )900 i), l750 C.

(Decom- 1 poses) g The above table shows the particular advantages of silicon carbide for use as the material of construction of the side walls 8 of the cooling chamber. The walls, if made of silicon carbide may, however, be made thinner than when made of fireclay because of the greater mechanical strength of silicon carbide at higher temperatures. The thinner walls of silicon carbide have a still greater power of transferring heat from the ware to the cooling air than is the case with silicon carbide walls of the same thickness as that of fireclay; for example; silicon carbide walls 11/2 inches thick (between the air ducts and the tunnel) would have a power of heat transmission according to the above data about 27 times the transmission of fireclay walls of `41/2 inches corresponding thickness and would still be stronger than the fireclay walls. lVhile fused alumina does not have as great thermal conductivity nor as great mechanical strength at high temperature as silicon carbide, it is still much superior to fireclay in respect to both of these properties. Fused alumina is, like fireclay, a highly oxidized refractory. The walls 8 adjacent to the cooling chamber are therefore made of a material whose thermal conductivity is at least greater than 0.006 oal irie/c1ii.3/sec./C. with due reference to other physical properties such as those mentioned above. One should consider, for ex ample, thermal emissivity and absorptive power which silicon carbide has in a particularly high degree. The outer wall 59 situated on the outside of the air ducts can be made of fireclay or other non-conducting refractory, the principal function of the wall 59 being to prevent undue loss of heat from the air that passes through the duct 10 (to be mixed with fuel in the burners or used in preheating the ware before it enters the combustion chamber) and to afford inechanical support for the walls of the cooling chamber.

As shown in Figure 8, the air circulates from a. point near the bottom of the kiln up the passages 10, through the openings l1 arranged along the top of the outer shell into a collector 12 that extends alon r the top of the cooling zone of the kiln. 'l he collector 12 communicates through a port or passage 13 with a longitudinally extending conduit 14, the conduit 14 extending along the greater portion of the length of the kiln. As shown in Figure 1 the conduit 14 is provided with lateral branches at 15, 16 and 17. The lateral branch 15 communicates with. a vertical passage 18 shown in detail in Figure 5, and each of these vertical passages communicates at its lower' end with the burner end of one of the combustion units 0. A damper'plate 19 is provided for controlling the iow of air through the passages 18. In a similar way air is conducted through similarly arranged passages through the lateral branches 1G to the other combustion units 6 at the opposite end of the Zone c. Air travels through the lateral branches. i7 down vertical passages 20 to the burner end of the combustion units 4 in the Zone in the event that the amount of preheated air supplied from the ducts l0 in the walls of the cooling chamber is insufficient for the amount of combustion required in the burner adjacent the liring chamber and preheating chambers` additional preheated air may be supplied through the ducts 50 which are located under the radiating combustion chambers 0. Since the walls of the radiating combustion chamber are commonly made of silicon carbide the air in the ducts 50 below the radiating combustion chamber 6 is highly heated by conduction. The amount of air passing through the duct 50 is partly regulated by induction exerted by the iiow of fuel through the orifice 7 into the combustion chamber 4.- The operator of the tunnel kiln may also contribute to the regulation by using the damper 51 in the duct 50.

The cooling effect of air depends, among other things, on its velocity. In certain tests it was found that between velocities of ten and sixty feet pery second the rate of cooling varied almost directly with the velocity of the air. In the case of thick iireclay Walls, however, such an effect is largely masked, and is very much smaller than in the case of silicon carbide Walls.

While metals have higher thermal conductivities than non-metallic refractory materials, commercially available metals will not stand the high temperatures of the tunnel kiln in the portions of the cooling zones which are adjacent to the firing zone.

The air which is preheated in passing through the Walls of the cooling chamber is utilized in supportin combustion in the various radiating com ustion units 4 and 6, thereby effectively increasing the operating temperatures of these units and the eiiiciency of the kiln.

The products of combustion from the radiating combustion units 6 and 4 are preferably discharged directly into the tunnel 3 and move along the kiln in a direction counter to the movement of the Ware through the tunnel. The invention in its broader aspects is not confined, however, to a structure Wherein the products of combustion are discharged into the tunnel, but also applies to cases wherein the products of combustion are separated at all times from the tunnel by a heat transmitting partition, as in the ordinary muiile kiln.

The radiating combustion units 4 and 6 may be of any suitable size and construction and may, for instance, be of the type disclosed in the United States Hawke Patent, No. 1,594,834, of August 3, 1926, wherein there is a double passage for the flame or may have only a single pass. The discharge ports from the radiating combustion chamer may be arranged to discharge directly into the tunnel or the discharged gases may be carried in ducts along the sides of the preheating zone in order to heat up the ware gradually Without direct contact therewith. In order to be effective, the radiating combustion units must be of a material Whose thermal conductivity is in excess of .006

cal./cm.3/sec./C. and of a material which move the mechanically held water from the ware. The atmospheric composition of the gases in this portion of the tunnel is not important, though it is important that the moisture content be fairly low and the circula# tion of gases be good. In some instances this zone might for convenience be separate from the remaining portion of the kiln, although we have shown it as being a part of the mam kiln structure.

In the second zone Zi the ware is heated to a much higher temperature than it is in the lirst zone. For instance, in the burning of some wares the heat may be such that the ware will be heated to a bright red. This zone is a relatively long one and the material preferably remains in this zone for a considerable period of time.

In the last heating zone c the temperature is brought up to the highest point to which it is desirable to fire the ware, and the atmosphere is maintained in the condition most desirable for the Ware in question. The ware, While it is in this zone, is immediately alongside of the combustion chambers, and from the combustion chambers the gases flow into the kiln and along the kiln, as previously described, although if desired the gases can be removed at any point by the provision of flues, as is well understood in the art. The atmosphere in this zone may be oxidizing, reducing or neutral, according to the character of the Ware being fired.

A radiating combustion chamber lends itself effectively to the control of the kiln atmosphere. For instance, the fuel mav be introduced into the combustion unit in such a Way that complete combustion will not be effected entirely Within the chamber, but the gases or flame coming from the chamber will contain considerable unburned fuel whose combustion will be completed in the kiln. In such a case a reducing atmosphere is maintained in the last heating zone. On the other hand, combustion, of the fuel may be made complete within the combustion unit, with little excess air so that the gases are substantially neutral. By burning the fuel completely in the combustion units in conjunction with an excess amount of air which is highly heated in the combustion units, the atmosphere in this zone of the kiln may be highly oxidizing. After the ware passes through the zone c it enters the cooling zone and is cooled, and as hereinbefore described, the heat which the ware gives oli is absorbed by the air iow ing through the passage 10. In order that the cooling of the Ware may be properly controlled the passages 10 are preferably arranged in groups along the walls of the cooling zone. Such groups are indicated at m, y and a in Figure l and any suitable means, such as` dampers (not shown) may be employed for controlling the amount of air circulated through the passages in addition to the regulation provided by the damper 49 in the passage 48. A principal advantage of the applicants arrangement for cooling the Ware (by radiation to conducting Walls Whose temperatures are regulated by flow of cold air through ducts therein) consists in the avoidance of injury to the Ware of the character which it suffers when the cooling is eiiected by the projection of cold air on the articles. In the case of ceramic Ware containing silica, sudden cooling may cause the silica to pass through an inversion point in which a portion of the silica changes from the amorphous form to one of the crystalline modilications of silica or from one crystalline modification to another. Since the different modifications of silica have very different coefficients of enpansion, the eii'ect of sudden air cooling is often very destructive to the Ware. The oxidizing action of the air on Ware which has just left the firing zone is also destructive to ware which contains carbon (free or combined), such for example as silicon carbide bricks.

Another objection to the cooling of the ware by direct impact arises from the fact that such cooling air in general strikes the outside objects first, While the inside objects are not cooled to anything like the same extent. Cooling b v radiation and conduction is of a more uniform character than cooling by cold air blasts especially where the ware is stacked in such a Way as to permit radiation from the inside pieces through openings between the outside pieces. As compared with a tunnel kiln in which the Walls of the cooling chamber are made of fireclay enclosing air ducts, the applicants are able to greatly shorten the cooling zone by the use of Walls constructed of material whose thermal conductivity is greater than 0.006 calorie/cm.3/sec./C. Such shortening of the cooling zone makes it possible to save room in the industrial plant and cuts down certain construction and maintenance costs. The shortening of the cooling zone, particularly when silicon carbide Walls are used, is made possible bythe fact that the thermal conductivity and emissivity ot silicon carbide are so high that effective cooling of the Ware by radiation and conduction can be obtained in a length of the tunnel which would be much too short to give sulicient cooling through lireclay walls containing air ducts. lith Walls of the last-mentioned character it is very diicultto limit the firing operation as applied to the Ware (except by subjecting them to blasts of cold air), because the heat of the firing zone extends itself by radiation and convection to the portions of the tunnel beyond the firing zone. Regulation by means of air flowing through ducts in ireclay walls is very difficult, and practically impossible Within a short length of the tunnel because of the poorvthermal 'conductivity of lireclay.

On account of the good ythermal conductivity of silicon carbide and on account of the thin layer of this material which it is possible to use between the cooling zone and the air ducts, air at very high temperatures can be supplied to the burners, whereby higher temperatures can be obtained in the radiating combustion chambers. Since the radiating power of a hot body is proportional to the -t'ourth power of its absolute temperature (Stefan s law) the importance of highly preheated air for radiating combustion chamber is obvious.

T he Walls of the different sections of the radlating combustion chamber can be made of different refractory materials each of which has a thermal conductivity greater than 0.000 calorie/cm.'/sec./C. For example, the section a: can be made of fused alumina on account of its high resistance to oxidation at high temperatures. The sections y and a can be made of bonded silicon carbide on account of its higher thermal conductivity, which is especially desirable in certain cases at the lower temperatures prevailing in these sections of the kiln.

iVhile the advantages of our cooling zone construction have been described so far in connection with continuous tunnel kilns, a similar method of cooling may be used n connection with periodic kilns". ln this case, the air which is heated by conduction through the Walls of the cooling zone is used for the heating of drying chambers for the Ware or for the preheating of Ware which is to be introduced later into a tiring chamber.

In the case where silicon carbide is used in vthe construction of the cooling Walls, the silicon carbide grains can be bonded with materials which are particularly non-reactive with silicon carbide at high temperatures and which are resistant to oxidation at such temperatures as Well as to reducing conditions'. Bonds of this character' are described in copending application Serial No. 269,075, filed April 11, 1928.

le claim l. In a cooling zone for a tunnel kiln, a plurality of sections having inner Walls adjacent to the tunnel and composed of materials Whose thermal conductivity is greater than 0.006 calorie/c1n.3/sec./C., the sectional Wall which is nearest the firing zone being composed of a stable and non-oxidizable refractory and the remaining sectional Walls being composed of bonded silicon carbide, air ducts in said sectional walls, and means for passing air at a controlled rate through said ducts to regulate the cooling effect of said Walls.

2. In a tunnel kiln a firing zone, a cooling zone through which ware under treatment passes after leaving the tiring Zone, the Walls of said cooling zone adjacent to the tunnel being composed of refractory materials Whose thermal conductivities are greater than 0.006 ca.lorie/cm.8/sec./C., said cooling zone being composed of a plurality of sections each provided with ducts for conducting currents of cooling air inside the Walls adjacent the tunnel, one of the sectional cooling Walls being composed of Stable, non-oxidizable refractory, While another sectional cooling Wall is composed of silicon carbide, means for supplying cool air under pressure to said ducts in each. section7 and dampers in'said ducts for controlling separately the rate of flow of cooling air through said ducts.

3. A sectional cooling zone for tunnel kilns in Which a section of the Zone subjected to higher temperatures is composed of a stable, non-oxidizable refractory material, while a section of the zone subjected to lower temperatures is composed of silicon carbide, said hotter and cooler sections being each supplied With ducts Jfor currents of cooling air in which the rates of flow are independently regulable.

In testimony whereof we aix our signatures.

EDVIN B. FORSE. CHARLES F. GEIGER. 

